Sodium carbonate (soda ash) is approximately the 11th highest volume chemical produced in the United States. It is used in the manufacture of glass, chemicals, soaps and detergents, and aluminum. It is also used in textile processing, petroleum refining, and water treatment.
For many years, sodium carbonate was produced by the Solvay process in which carbon dioxide was dissolved in water containing ammonia (NH.sub.3) and salt (NaCl) to precipitate sodium bicarbonate which was then separated by filtration and heated to form sodium carbonate. Because of high energy costs and problems with disposing of chloride-containing waste streams generated by the Solvay process, it has been abandoned in the United States in favor of obtaining sodium carbonate from naturally occurring trona deposits. Trona deposits are located in Utah, California, and Wyoming. Green River, Wyo. contains the largest known trona deposits in the United States and is actively mined by five companies.
Crude trona ("trona ore") consists primarily (80-95 percent) of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.b.2H.sub.2 O) and in lesser amounts, sodium chloride (NaCl), sodium sulfate (Na.sub.2 SO.sub.4), organic matter, and insolubles such as clay and shales. In Wyoming, these deposits are located in 25 separate identified beds or zones ranging from 800 to 2800 feet below the earth's surface and are typically extracted by conventional mining techniques such as the room and pillar and longwall methods. The cost of these conventional mining methods is high, representing as much as 35 percent of the production costs for soda ash. Furthermore, recovering trona by these methods becomes more difficult as the best, most thickly bedded trona deposits are depleted. As a result, recovery of carbonate values from trona has fallen in some cases by as much as 5 to 7 percent. Development of new reserves is expensive, requiring a capital investment of as much as $100 to 150 million in 1995 dollars to sink new mining shafts and to install related equipment.
As its chemical composition indicates, trona ore requires processing in order to recover the sodium carbonate. Most of the sodium carbonate from the Green River deposits is produced from the conventionally mined trona ore via the "monohydrate" process. The "monohydrate" process involves crushing and screening the trona ore which, as noted above, contains both sodium carbonate (Na.sub.2 CO.sub.3) and sodium bicarbonate (NaHCO.sub.3) as well as impurities such as silicates and organic matter. After the trona ore is screened, it is calcined (i.e., heated) at temperatures greater than 150.degree. C. to convert sodium bicarbonate to sodium carbonate. The crude soda ash is dissolved in a recycled liquor which is then clarified and filtered to remove the insoluble solids. The liquor is sometimes carbon treated to remove dissolved organic matter which may cause foaming and color problems in the final product, and is again filtered to remove entrained carbon before going to a monohydrate crystallizer unit, an evaporator system generally having one or more effects (evaporators), where sodium carbonate monohydrate is crystallized. The resulting slurry is centrifuged, and the separated monohydrate crystals are sent to dryers to produce soda ash. The soluble impurities are recycled with the centrate to the crystallizer where they are further concentrated. To maintain final product quality, it eventually becomes necessary to remove the impurities with a crystallizer purge stream.
The production of sodium carbonate using the combination of conventional mining techniques followed by the monohydrate process is becoming more expensive as the higher quality trona deposits become depleted and labor and energy costs increase. As stated above, recovery of sodium carbonate (usually expressed as tons of sodium carbonate produced per ton of trona ore) has fallen as the higher quality reserves have been mined. Furthermore, the costs of developing new reserves requires substantial capital investment, as much as $100-150 million in 1995 dollars.
Recognizing the limitations of conventional mining techniques, various solution mining techniques have been proposed. Solution mining allows the recovery of sodium carbonate from trona deposits without the need for sinking costly mining shafts and employing workers in the mines. In its simplest form, solution mining comprises injecting water (or an aqueous solution) into a deposit of soluble ore, allowing the solution to dissolve as much ore as possible, pumping the solution to the surface, and recovering the dissolved ore from the solution.
For example, a solution mining technique was proposed in U.S. Pat. No. 2,388,009 to Pike on Oct. 30, 1945. Pike discloses a method of producing soda ash from underground trona deposits in Wyoming by injecting a heated brine containing substantially more carbonate than bicarbonate which is unsaturated with respect to the trona, withdrawing the solution from the formation, removing organic matter from the solution with an adsorbent, separating the solution from the adsorbent, crystallizing and recovering sodium sesquicarbonate from the solution, calcining the sesquicarbonate to produce soda ash, and re-injecting the mother liquor from the crystallizing step into the formation.
A second patent to Pike, U.S. Pat. No. 2,625,384, discloses another solution mining method which uses water as a solvent under ambient temperatures to extract trona from existing mined sections of the trona deposits. The subsequent solution is recovered from the mine and heated before dissolving additional dry mined trona in it, forming a carbonate liquor which can subsequently be processed into sodium carbonate.
As an additional complicating factor, however, sodium carbonate and sodium bicarbonate have different solubilities and dissolving rates in water. These incongruent solubilities of sodium carbonate and sodium bicarbonate can cause bicarbonate "blinding" when employing solution mining techniques. Blinding can slow dissolution and may result in leaving behind significant amounts of reserves in the mine. Blinding occurs as the bicarbonate, which has dissolved in the mining solution, tends to redeposit out of the solution onto the exposed surface of the ore as the carbonate saturation in the solution increases, thus "blinding" this surface--and its carbonate values--from further dissolution and recovery.
U.S. Pat. No. 3,184,287 to Gancy discloses a method for preventing bicarbonate blinding in the mine by using an aqueous solution of an alkali, such as sodium hydroxide having a pH greater than sodium carbonate, as a solvent for solution mining. U.S. Pat. No. 3,953,073 to Kube and U.S. Pat. No. 4,401,635 to Frint also disclose solution mining methods using a solvent containing sodium hydroxide. U.S. Pat. No. 5,043,149 discloses a process for disposing of insoluble tailings that remain when solubilizing uncalcined or calcined trona in the process of producing soda ash in which the tailings are slurried with water or waste solutions of sodium carbonate or sodium bicarbonate or both, injected into an underground mined out cavity and removing a liquor from the cavity whose concentration of sodium carbonate or sodium bicarbonate or both has been increased and from which sodium-based chemicals may be recovered.
A significant problem associated with solution mining is the subsequent recovery of the sodium carbonate from the relatively low concentration of carbonate and bicarbonate in the solution mine brine. In addition, the solution mining techniques disclosed above produce brines containing sufficient sodium bicarbonate and other impurities to prevent processing into sodium carbonate by the conventional monohydrate process. A major problem is the co-precipitation of sodium sesquicarbonate crystals during sodium carbonate monohydrate crystallization which reduces the quality of the final product.
It is well known to those skilled in the art that sodium bicarbonate can be converted to sodium carbonate by heating the sodium bicarbonate to a sufficiently high temperature. For example, U.S. Pat. No. 2,133,455 issued to Keene et al. discloses a method of converting a solution of sodium bicarbonate to sodium carbonate by stripping the sodium bicarbonate solution in a tower with steam having a temperature above 100.degree. C. The resulting carbonate solution is then mixed in an evaporator with a saturated solution of sodium carbonate containing some dissolved sodium chloride.
U.S. Pat. No. 3,113,834 to Beecher et al. discloses a process for producing dense soda ash which includes the steps of decomposing a sodium bicarbonate solution by heating it to its boiling point while it is under pressure in excess of atmospheric. Beecher et al. discloses temperatures for decomposing sodium bicarbonate solutions between 150.degree. C. to about 250.degree. C. at super atmospheric pressure. U.S. Pat. No. 3,451,767 to Saeman et al. also discloses a method of making dense soda ash by decomposing a sodium bicarbonate solution at 150.degree. to 250.degree. C. and at 80 to 500 psia (pounds per square inch absolute) to form water, carbon dioxide, and anhydrous sodium carbonate. According to the Saeman process, sodium bicarbonate is mixed with a recycle stream containing suspended sodium carbonate seed crystals, a step not disclosed in Beecher. Similarly, Fujita et al. discloses a process for preparing sodium carbonate anhydride by countercurrent contacting of high pressure steam and a suspension of sodium bicarbonate or sodium sesquicarbonate in a concentrated solution of sodium carbonate at temperatures above 150.degree. C.
Miller, in U.S. Pat. No. 3,264,057, discloses a process for producing soda ash crystals by leaching carbonate values from crude trona and decomposing the major part of sodium bicarbonate in the resulting solution by steam stripping at about 110.degree. C. Miller also discloses in U.S. Pat. No. 3,246,962 a method for obtaining concentrated solutions of sodium carbonate and sodium bicarbonate from crude mined trona by leaching the alkali values from crude trona in a column while passing steam into the column.
Copenhafer et al. discloses in U.S. Pat. No. 5,283,054 a method for producing soda ash from a brine solution containing sodium carbonate and sodium bicarbonate by heating the brine to about 100.degree. C. to about 140.degree. C. to evaporate water, convert sodium bicarbonate to sodium carbonate, and to drive off resulting carbon dioxide and reacting the brine with reduced sodium bicarbonate with an aqueous sodium hydroxide solution in amounts to convert essentially all of the remaining sodium bicarbonate in the brine to sodium carbonate. The resulting sodium carbonate solution is further processed to recover soda ash.
Frint and Copenhafer, in U.S. Pat. No. 5,262,134, disclose a process for producing soda ash from a brine solution containing sodium carbonate and sodium bicarbonate by heating the brine to between 90.degree. C. and 115.degree. C. to evaporate water, convert sodium bicarbonate to sodium carbonate, and to drive off the resulting carbon dioxide therefrom until the concentration of sodium carbonate and sodium bicarbonate in the brine form a solution that will crystallize sodium sesquicarbonate. The resulting solution is then processed into various sodium based chemicals including sodium carbonate.
All of these disclosed processes require steam and temperatures of at least 90.degree. C. Because the production of steam is expensive, a process which avoids the need to use steam or can make use of steam already available would allow the recovery of sodium carbonate chemicals from solution mining in an economical fashion without off-setting the costs saved through solution mining by higher energy costs in connection with processing the solution mine brine into soda ash or other sodium based chemicals. A process not requiring the generation of additional steam, other than is normally generated for the production of sodium based chemicals, would further reduce energy costs. Also, many of the prior art processes, particularly those being commercially developed, require the use of substantial quantities of expensive neutralizing agents such as caustic soda and lime. A process which eliminates or reduces the need for these reagents would greatly reduce production costs.